Naturally occurring bacteria are known to catalyze the dissolution of minerals. It is widely accepted that certain chemoautotrophic bacteria, such as Thiobacillus ferrooxidans among others, catalyze the oxidation of ferrous ion (Fe.sup.+2) to the ferric (Fe.sup.+3) state and sulfide (S.sup.= or S.sup.-) to sulfate (SO.sub.4.sup.=), and utilize these low energy reactions for producing metabolic energy for growth. Fe.sup.+3 is a chemically reactive ion which oxidizes many metal or sulphur bearing materials. In the process, the ferric ions are reduced to the Fe.sup.+2 state and the sulfate reacts with water to lower the pH. In essence, these types of bacteria are able to generate a highly oxidative, acidic solution which is conducive to the dissolution of numerous materials such as those containing copper, iron, zinc, lead and mercury.
The leaching of materials such as ore using such chemoautotrophic organisms, a process termed bioleaching, has attracted much attention and interest in recent years due to increasing environmental awareness and decreasing ore grades. In some situations, it is no longer economically feasible to smelt low grade ores. Bioleaching is a more cost effective means of recovering metal from lower grade ores in that bioleaching is less energy consumptive and presents a lower environmental risk.
Some controversy exists as to how chemoautotrophic bacteria accomplish bioleaching. Current mining industry belief is that direct attachment of the bacteria to the ore is critical in leaching the ore. The bacterial adhesion is thought to be the initial step for oxidation. Most current bioleaching techniques involve the acidification of an inoculum of chemoautotrophic bacteria introduced to a pile of ore. Sulfuric acid and nutrients are then added to the ore to encourage the organism to oxidize the minerals below.
The problem with such current approaches to bioleaching is that toxins native to the ore will be solubilized during the oxidation process. Such toxins include arsenic, mercury, cadmium and other metals and metalloids. These metals and metalloids at low concentrations will destroy the bacteria resulting in significant down-time waiting to acclimate new bacteria to the ore pile. Further, the chemoautotrophic bacteria utilized in bioleaching are typically mesophilic and grow at temperatures between 25.degree. C. and 40.degree. C. Bioleaching reactions are exothermic in nature and, as a result, much heat is released so that the center of the ore pile may reach temperatures as high as 90.degree. C. Accordingly, the bacterial oxidation activity can only occur in the cooler, top layer of the ore pile.
Pile bioleaching as well as other processes involving bioleaching, often present harsh conditions for optimal bacterial activity. Bioleaching has long been treated as a single unit process making optimization of the process a difficult task. Changes in the conditions within the pile found to be advantageous in conventional chemical leaching may adversely affect the activity of the organism in bioleaching.
Since their discovery, chemoautotrophic organisms useful in bioleaching were viewed by the mining industry as a promising and inexpensive means of oxidizing various components of crude ore. Although several large scale attempts have been made to enhance the growth of these organisms, little commercial success has been realized. Invariably as growth of the bacteria proceeds, the dissolution of toxins kill the organisms. In large scale operations, this leads to lost productivity as new bacteria must be reintroduced and established in the leach pile. In addition, bacterial activity drops considerably during the cold weather months. While bioleaching with chemoautotrophic bacteria has been known for many years, commercial applications have yet to be adopted as a viable industrial process due to the above problems.
Leaching can also be accomplished by allowing contact of the ferric ion solution, produced chemically, with the material to be treated. This process, however, has not proven commercially feasible due to the prohibitive cost of purchasing or chemically producing the ferric ion solution.